In the paper industry, fillers are presently suspended in water together with the fibrous materials and auxiliary substances. They come into contact with rotating and stationary parts or get between surfaces moving relative to one another on machine elements, such as, for example, suction device coatings and the screen of a paper machine.
In order to determine numerical data concerning the wearing effect of fillers in aqueous suspension in the cellulose and paper industry, as well as to determine wear in cellulose or pulp and paper machine screens and in the suction device coatings (ceramic, plastic and the like) employed therein, an abrasion testing apparatus has been developed (German Patent Specification No. 20 33 592) which is being used worldwide in the meantime under the name "AT 1000". The testing method special to this abrasion testing apparatus is defined in Leaflet V/27.5/75 issued by Verein der Zellstoff--und Papier-Chemiker und--Ingenieure (ZELLCHEMING) (Association of Cellulose and Paper Chemists and Engineers) and has thereby been acknowledged as obligatory by the relevant industry. In this abrasion testing apparatus, a standard test screen of phosphor bronze is employed as the test piece.
With the increased use of plastic screens instead of bronze screens in the cellulose and paper industry, the desire to use a standard plastic screen instead of the standard bronze screen has become ever stronger. Extensive investigations have shown, however, that when plastic screens are employed, no reproducible and significant test results can be obtained. It has been found, in fact, that certain fillers which showed a very slight abrasion effect with the use of bronze screens already led in part to complete destruction on plastic screens before the complete duration of the test. An additional aggravating factor as regards a possible choice of a standard plastic screen for determining abrasion is the fact that, depending on the quality of paper, not only are screens of varying fineness used as warp or weft runners, but also multi-layer screens have been tried out in practice. Finally, investigations have also shown that the surface roughness of a rotary ceramic body employed in the known testing apparatus exerts a great effect on the wear of the plastic screen. In order to get closer to conditions in practice, an abrasion testing apparatus has been developed experimentally (Wochenblatt der Papierfabrikation (Paper Manufacture Weekly) 9, 1981, page 295) which has essentially the structure mentioned at the beginning. In this apparatus, the driving shaft of the drum is arranged horizontally and eccentrically in a substantially cylindrical tank or container. The tank has an opening at the side. The retaining and tensioning device is provided in the region of this opening, this device having a chamber closed by a rubber diaphragm towards the circumference of the drum. By introducing compressed air into this chamber, the plastic screen held by the retaining and tensioning device can be pressed more or less strongly against the circumference of the drum. The tank has an inlet connection so that filler suspension can be fed into the crescent-shaped space between the drum and the wall of the tank. This filler suspension is intended to be entrained by the rotating drum, so that it gets between the surfaces in contact with each other, namely, between the suction device coatings arranged on the drum and the plastic screen pressed against the drum. This known abrasion testing apparatus, however, is comparatively costly to manufacture because of the complicated retaining and tensioning device. Moreover, a pump which keeps delivering the filler suspension back into the tank must be provided. Due to the horizontal arrangement of the driving shaft, considerable sealing problems moreover arise in those places where the driving shaft emerges from the tank and also where the tank has a lateral opening for the plastic screen. It is also not guaranteed that the filler emulsion will actually reach the zone between the suction device coatings and the plastic screen in sufficient quantity. Furthermore, the contact pressure of the plastic screen is not exactly reproducible and therefore, also, no reproducible test results can be obtained.
The problem underlying the invention is to provide an abrasion testing apparatus of the kind mentioned at the beginning which, together with simple design and long life, guarantees test conditions close to conditions obtained in practice and also enables acquisition of reproducible and comparable results.